lowest-common-ancestor-of-a-binary-search-tree.js

/**
 * Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
 *
 * According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes
 * v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”
 *
 *         _______6______
 *        /              \
 *     ___2__          ___8__
 *    /      \        /      \
 *    0      _4       7       9
 *          /  \
 *          3   5
 *
 * For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2,
 * since a node can be a descendant of itself according to the LCA definition.
 */

/**
 * @param {TreeNode} root
 * @param {TreeNode} p
 * @param {TreeNode} q
 * @return {TreeNode}
 */
const lowestCommonAncestor = (root, p, q) => {
  if (!root) {
    return null;
  }

  if (root.val > p.val && root.val > q.val) {
    return lowestCommonAncestor(root.left, p, q);
  }

  if (root.val < p.val && root.val < q.val) {
    return lowestCommonAncestor(root.right, p, q);
  }

  return root;
};

export default lowestCommonAncestor;